The Business FAQ page is truly comprehensive. Do a keyword search or browse by category.
Knowing if a company is publicly traded makes a big difference in understanding what kind of information you can expect to find about it. While publicly traded companies must report financial information to government regulatory agencies (e.g., the SEC), private companies don't need to disclose much at all.
If you aren't sure about a company, try using the company search in LexisNexis. You'll see in the list of results whether a company is public or private.
If a company is private...
If a company is public...
When companies file their financial disclosure statements with the SEC, the reports end up in a database called EDGAR, which stands for Electronic Data Gathering, Analysis, and Retrieval System. You have the option to search EDGAR via the SEC's website or via subscription library databases.
Simply put, these are two industry classification systems and as you begin to navigate the business databases, you will see them everywhere.
SIC is an older, more international system, still in use by the U.S. Government.
NAICS is a little newer and generally applies to North America. Either way, don't fret too much about these. Just understand that they are used to code specific industries, largely for the purposes of research and statistics.
For a more complete explanation, see the section on classifications in this article from the International Encyclopedia of the Social & Behavioral Sciences.